This invention relates to apparatus for controlling the rotational speed of a recording disc, and more particularly to apparatus of the type which controls the rotational speed of the recording disc to a constant linear velocity at any signal pick-up point on the recording disc.
PCM (Pulse Code Modulation) signal recording and reproducing systems have been intensitively developed recently. This kind of system is used to convert analogue information such as an audio signal into digital information by PCM and then record it on a disc forming "pits" consisting a spiral track of the disc. The recorded information is reproduced by, for example, irradiating a convergent light beam such as a laser beam to the track of the disc and detecting the variation of the returned beam therefrom.
There are known two types of systems for controlling the rotational speed of the disc. One is a constant rotational speed system which rotates the disc at an uniform speed, and the other is a constant linear velocity system which causes the linear velocity of the disc to be constant at any signal pick-up point on the disc. The constant rotational speed system is suitable for use in recording and reproducing a signal having periodicity such as a video signal because of possibility of special reproducing operation such as stillness, slow motion and quick motion.
The constant linear velocity system is more suitable for use in recording and reproducing an audio signal which does not need the above-mentioned special reproducing operation because the signal is recorded with uniform density at any track on the disc and a large capacity of signal may be recorded.
Japanese Patent Disclosure No. 10707/79 describes a rotational speed controlling apparatus embodying a constant linear velocity system. This prior art is provided with a potentiometer. The potentiometer detects the position of the signal pick-up head on the disc and produces the voltage signal V.sub.R in proportion to the distance from the center of the disc. The rotational speed of the disc is controlled in response to the voltage signal V.sub.R. This prior art, however, fails to rotate the disc with an accurately constant linear velocity, because the accuracy of detecting the position of the head depends on the accuracy of the potentiometer.